1. Field of the Invention
The present invention relates to an exhaust gas purification device for an internal combustion engine. Specifically, it relates to a structure of an HC trapping catalyst that adsorbs and purifies unburned fuel (RC) in exhaust gas.
2. Description of the Related Art
In an exhaust passage of an internal combustion engine, a ternary catalyst for removing HC, CO, and NOx has been widely adopted. Since the capacity of the ternary catalyst remarkably decreases at low temperature, as exhaust gas purification devices, those further comprising an HC trapping catalyst provided in the exhaust passage have been increasingly employed.
The HC trapping catalyst comprises an HC adsorbing layer, which adsorbs HC at low temperature and discharges HC at high temperature, and a ternary catalyst layer and has a function of preventing discharge of HC which cannot be removed in the ternary catalyst at cold starting of the internal combustion engine, the function being achieved by oxidative removal of HC, which has been adsorbed in the HC adsorbing layer at low temperature, in the ternary catalyst layer at high temperature (refer to JP-A-2004-9029).
However, in the HC trapping catalyst as described above, the ternary catalyst layer is formed on the HC adsorbing layer and the HC adsorbing layer is in close contact with the ternary catalyst layer, so that the HC adsorbing layer is heated and its temperature is steeply elevated when the oxidation of HC starts in the ternary catalyst layer. Accordingly, HC adsorbed in a large amount immediately after the cold starting is rapidly discharged from the HC adsorbing layer and thus there is a possibility that HC is not completely oxidized in the ternary catalyst layer and is discharged from the HC trapping catalyst.